In a long-distance telephone line via a submarine cable or via a communication satellite, the subscriber's line, in general, connected to both ends of the line is of a two-wire circuit and its long-distance transmission portion is of a four-wire circuit for the purposes of amplification of a signal, etc. Similarly, in the mobile communications network using a mobile telephone (or cellular phone), the subscriber's line of a terrestrial analog telephone is of a two-wire circuit and its portion from a terminal of the mobile telephone to a switch, etc. is of a four-wire circuit. In this case, the connection region between the two-wire and the four-wire is provided with a hybrid circuit for performing a four-wire/two-wire conversion.
This hybrid circuit is designed to match with the impedance of the two-wire circuit. However, since it is difficult to obtain always a good matching condition, a received signal reaching an input side of the four-wire of the hybrid circuit tends to leak toward an output side of the four-wire, thereby generating a so-called echo. Since such an echo is lower in level than the talker's voice and reaches the talker after a delay of a predetermined time period, a speech hindrance is created. Such a speech hindrance caused by echo becomes more significant as the signal propagation time becomes longer. Particularly, in the case of a mobile communication using a mobile telephone, since various processing procedures are carried out in the radio communication section leading to the switch, etc., the delay of signal is increased, thus resulting, particularly, in the problem of speech hindrance caused by echo. FIG. 2 shows one example of the waveform of an echo with respect to impulse response.
As an apparatus for preventing a generation of echo, there are an echo suppressor and an echo canceler. FIG. 1 shows a schematic construction of an echo canceler which can be used in a mobile communications network. The echo canceler 1 illustrated here is located on a front stage of a hybrid circuit 2. In this illustration, the subscriber of an ordinary analog telephone is referred to as the "near-end talker" and the subscriber of a mobile telephone as the "far-end talker". A far-end voice signal input into the echo canceler 1 is represented by Rin; a far-end voice signal output from the echo canceler 1, by Rout; a near-end voice signal input into the echo canceler 1, by Sin; and a near-end voice signal output from the echo canceler 1, by Sout; respectively.
The echo canceler 1 shown in FIG. 1 comprises an echo path estimation/echo replica generation circuit 3, a control unit 4, an adder 5, and a non-linear processor 6. Here, the echo path estimation/echo replica generation circuit 3 detects a response characteristic of the hybrid circuit 2 based on both the far-end voice input Rin and near-end voice input Sin and estimates an echo path (namely, echo propagating line). Then, an anticipated echo (namely, echo replica) from the hybrid circuit 2 is generated through a convolutional operation based on the result of estimation and the far-end voice input Rin. This echo replica is generated by an FIR filter which is constituted of so many taps as 512, for example. A convolutional operation in an echo replica refers to this. In the adder 5, this echo replica is subtracted from the near-end voice input Sin, thereby canceling the echo. As the above-mentioned echo path estimation algorithm, a learning identification algorithm is used. Among many adaptive algorithms, this learning identification algorithm is comparatively small in computational complexity and good in convergence characteristic.
Specifically, the echo path estimation/echo replica generation circuit 3 has an FIR filter. An echo replica signal Y(z) output from the FIR filter can be obtained by the following equation (1). ##EQU1##
In the equation (1), N is the number of taps of the FIR filer, and b.sub.i (where i=0, 1, 2, . . . N-1) is a tap coefficient in each tap. If appropriate values of the tap number N and tap coefficient b.sub.i can be obtained by estimation of an echo path, the echo replica signal Y(z) is approximated to an actual echo. Thus, echo is canceled in the adder 5. As the above-mentioned echo path estimation algorithm, an adaptive filter technique, for example, a learning identification algorithm, which among many other adaptive algorithms, is comparatively small in computational complexity and good in convergence characteristic, is used. Details of the learning identification algorithm is disclosed, for example, in Institute of Electronics and Communication Engineers of Japan (IECE) Journal '77/11 Vol. J60-A NO.11, article under the heading of "Regarding Echo Canceling Characteristic of Echo Canceler Using Learning Identification Algorithm".
As conditions for enabling the above learning, the following requirements must be met.
1 A far-end voice output Rout of the level sufficient for an echo to come back as a near-end voice input Sin is present. In other words, the far-end taker is currently engaged in speech.
2 The near-end voice input Sin is constituted of an echo (or an echo and a white noise) alone. In other words, the near-end taker is not engaged in speech.
On the other hand, when the far-end talker is in a speechless condition and when the far-end talker and the near-end talker are simultaneously engaged in speech (this state is hereinafter referred to as the "double talk"), it is necessary to turn off the learning function because there is a fear to cause a mis-learning state of echo path estimation.
In the transmission line, digital signals are transmitted, and a D/A conversion (in a general expression, a .mu.-LAW conversion) is made between the echo canceler 1 adapted to process such digital signals and the hybrid circuit 2 adapted to undertake a conversion to the analog line. For this reason, a non-linear characteristic relation is established between the far-end voice output Rout and the near-end voice input Sin. Therefore, echo cannot be canceled fully and completely merely through the linear computation by means of the echo path estimation/echo replica generation circuit 3, etc. As a consequence, an echo component unable to be completely canceled is produced.
In order to remove such an echo component (hereinafter referred to as the "residual echo"), the non-linear processor 6 is employed. This non-linear processor 6 undertakes a non-linear switching operation. Specifically, in the case where the near-end voice output Sout is constituted merely of an echo, in other words, in the case where only the far-end talker is currently engaged in speech (this state is hereinafter referred to as the "far-end talker's single talk"), a switching operation is made such that the transmission of the near-end voice output Sout is prohibited or an operation is made such that the near-end voice output Sout is replaced by a pseudo noise.
The control unit 4 controls the echo path estimation/echo replica generation circuit 3 and the non-linear processor 6. That is, the control unit 4 detects the far-end taker's speechless condition or detects the double talk, controls the ON/OFF state of the learning function of the echo path estimation in accordance with a double talk signal DT, detects the far-end talker's single talk, and controls the switching operation of the non-linear processor 6.
Incidentally, in the above-mentioned techniques, there are encountered with the following problems.
1 Firstly, since the above-mentioned techniques merely employ an adaptive filter technology such as the learning identification algorithm, if the delay time of an echo to be canceled is increased, the number of taps of the adaptive filter is increased and the computational complexity is also increased.
In other words, the echo path estimation/echo replica generation circuit 3 estimates an echo path presuming that the far-end voice input Rin and the near-end voice input Sin are time-wise coincident with each other, and generates an echo based on the estimated echo path. However, since the near-end voice input Sin from the hybrid circuit 2 is delayed, with respect to the far-end voice input Rin, by a delay time attributable to a transmission path between the echo canceler 1 and the hybrid circuit 2, the far-end voice input Rin is input first in the echo path estimation/echo replica generation circuit 3 and then, the near-end voice input Sin corresponding to Rin is input therein with the above-mentioned delay time. During this time, it becomes impossible to satisfactorily carry out a learning based on the estimation of an echo path.
2 Also, in the above-mentioned conventional techniques, the echo canceler did not have any information of the echo path at the start of an operation. However, observation of the present inventors revealed that characteristic of an echo path is substantially controlled by characteristic of a hybrid. Specifically, the waveform of an echo determined by a hybrid was longitudinally shifted on the time axis in accordance with a delay in the transmission line and attenuated in accordance with the attenuation in the transmission line. As a result, a waveform of an echo in the transmission line with respect to an impulse input was obtained with a considerable accuracy.
3 A digital signal is transmitted in the transmission line and a D/A conversion (in general, .mu.-LAW conversion) is performed between the echo canceler 1 for processing the digital signal and the hybrid circuit 2 for performing a conversion to an analog line. For this reason, a linear relation is established between the far-end voice output Rout and the near-end voice input Sin. Therefore, it is impossible to fully and completely cancel the echo merely by linear operation using the echo path estimation/echo replica generation circuit 3, etc.
In order to improve the shortcomings of 1 to 3 as a group, it is necessary to change the design of the echo canceler extensively or to modify it entirely. This being the case, it was unexpectable that the existing equipment is effectively used. Further, recently, there is a demand of a high-speed convergence with respect to an echo canceler.